The invention disclosed herein relates to frequency and time generators and standards, to control thereof and to thermometry, barometery and other fields of measurement and instrumentation. More particularly, the invention relates to multimode oscillators, control thereof and to use thereof in frequency and time generators and standards and in thermometry, barometery and other fields of measurement and instrumentation.
The subject of dual-mode oscillators including crystal resonators which simultaneously energize, excite or stress two modes of the crystal resonator and simultaneously generate two frequencies is discussed in the literature. For example, such oscillators utilizing an SC-cut quartz crystal energized in the dual C mode were extensively discussed at the 43rd Annual Symposium on Frequency Control (May 22-25, 1989). See, for example, the following. Schodowski, Stanley S., "Resonator Self-Temperature-Sensing Using a Dual-Harmonic-Mode Crystal Oscillator", Pro. 43rd Ann. Symp. on Frequency Control, pp. 2-7; Filler, Raymond L. & Vig, John R., "Resonators for the Microcomputer Compensated Crystal Oscillator", Pro. 43rd Ann. Symp. on Frequency Control, pp. 8-15; Bloch, Martin, Meirs, Marvin & Ho, John, "The Microcomputer Compensated Crystal Oscillator (MCXO)", Pro. 43rd Ann. Symp. on Frequency Control, pp. 16-19; Benjaminson, A. & Stallings, S. C., "A Microcomputer-Compensated Crystal Oscillator Using A Dual-Mode Resonator", Pro. 43rd Ann. Symp. on Frequency Control, pp. 20-26; Filler, R. L., Messina, J. A. & Rosati, V. J., "Frequency-Temperature and Aging Performance of Microcomputer-Controlled Crystal Oscillators", Pro. 43rd Ann. Symp. on Frequency Control, pp. 27-33; Bloch, Martin, Meirs, Marvin, Ho, John, Vig, John R. & Schodowski, Stanley S., "Low Power Timekeeping", Pro. 43rd Ann. Symp. on Frequency Control, pp. 34-36.
Pro. 43rd Ann. Symp. on Frequency Control, pp. 2-7, cited above, describes double gain loop and single gain loop dual-mode oscillators capable of simultaneously generating two SC-cut C-mode frequencies. The double gain loop oscillator is described as two separate oscillators sharing a crystal that operates at series resonance, a condition obtained with series or parallel tuned networks that include both capacitors and inductors. Additional circuitry such as a frequency tripler, mixer and low pass filter may also be needed. This double gain loop oscillator has the disadvantages of employing a relatively large number of parts and employing inductors. Inductors exhibit hysteresis effects, require a relatively large amount of space and are difficult to form in integrated circuits as compared to other components such as capacitors.
The single gain loop, Colpitts-type oscillator described in Pro. 43rd Ann. Symp. on Frequency Control, pp. 2-7, also employs capacitors and inductors to provide the correct phase shift for oscillation at the two C-mode frequencies, and to suppress an unwanted B-mode frequency. The inherent non-linearity in a transistor provides for multiplication and mixing of the two C-mode frequencies to produce a beat frequency. A signal at the beat frequency and a signal at one of the two C-mode frequencies are provided as outputs from the oscillator. Although this single gain loop oscillator requires less input power and fewer components than the double gain loop oscillator described above, the single gain loop oscillator has the drawback that it does not permit separate control of the two crystal-mode currents (power), requires a singly or doubly selective amplifier and a low pass filter at its outputs, and also requires inductors.
Pro. 43rd Ann. Symp. on Frequency Control, pp. 20-26 discusses the need for the inductors in a double gain loop SC-cut dual mode oscillator. Also, when perpendicular field excitation resonators are employed, it is commonplace to employ narrowly tuned rejection networks or traps to prevent oscillation at unwanted frequencies; such traps ordinarily incorporate one or more inductors.
One use for such dual mode oscillators described in the literature is to provide compensation for microcomputer-controlled crystal oscillators, particularly temperature compensation. Temperature stability of crystal-controlled oscillators was frequently attempted by placing the crystal in an oven. See, for example, U.S. Pat. No. 4,748,367 (Bloch et al.), assigned to the assignee of this application, and U.S. Pat. No. 4,839,613 (Echols et al.). One problem associated with oscillators employing ovens is that such oscillators are not suitable for low power applications.